The present invention relates to an image sensor for use in such apparatuses as a facsimile machine and a scanner. More particularly, in an image sensor of the type which detects signals by matrix-driving using switching elements connected to respective light-receiving elements, the invention is directed to the removal of interference of a switching element control voltage with signal detection lines.
Conventional facsimile machines and the like use a contact-type image sensor in which an image of, e.g., a document is projected onto the image sensor with one-to-one magnification and then converted into electric signals. Further, TFT (thin-film transistor)-driven image sensors have been proposed in which the projected image is detected while being divided into a multiplicity of pixels corresponding to light-receiving elements; charges generated in the light-receiving elements are transferred to wiring capacitances through switching elements formed of a TFT on a block-by-block basis and temporarily stored therein; and the stored charges are sequentially read out as electric signals by a driver IC at a rate of several hundreds of kilo-hertz to several mega-hertz. The matrix operation of the switching elements allows image information to be read out by a single driver IC, thus contributing to the reduction of the number of driver ICs for the image sensor.
FIG. 9 shows an example of a TFT-driven image sensor, and FIG. 10 shows an equivalent circuit for a single bit part of this image sensor. The image sensor includes: a light-receiving element array 101 in which a plurality of light-receiving elements P.sub.1,1 -P.sub.k,n are arranged in line over a length substantially the same as the width of a document; a charge transfer section 102 consisting of a plurality of switching elements T.sub.l,l -T.sub.k,n which are associated with the respective light-receiving elements P.sub.l,l -P.sub.k,n ; and a matrix-like multilayer wiring 103.
The light-receiving element array 101 is divided into k blocks, each block consisting of n light-receiving elements P.sub.i,l -P.sub.i,n. Each light-receiving element P.sub.i,j can be represented equivalently by a photodiode PD and a parasitic capacitance C.sub.p. Each light-receiving element P.sub.i,j j is connected to the drain electrode of the corresponding switching element T.sub.i,j. The source electrode of each switching element T.sub.i,j is connected to a common signal line 104 (a total of n lines), which is commonly used for the k blocks, through the matrix-like multilayer wiring 103, and each common signal line 104 is connected to a driver IC 105. A TFT control circuit 106 is connected to the gate electrodes of the switching elements T.sub.l,l -T.sub.n,k such that the switching elements are made conductive on a block basis.
A charge photoelectrically generated in each light-receiving element P.sub.i,j is stored in the parasitic capacitance C.sub.p of the light-receiving element and the drain-gate overlap capacitance C.sub.GD of the corresponding switching element T.sub.i,j for a certain period, and is thereafter re-distributed to the wiring capacitance C.sub.L of the multilayer wiring 103 and the common signal line 104 and the source-gate overlap capacitance C.sub.GS of the switching element T.sub.i,j on a block basis using the switching element T.sub.i,j as a switch for transferring the charge. For example, when a gate pulse G.sub.l sent via a control line G.sub.l from the TFT control circuit 106 turns on the switching elements T.sub.l,l -T.sub.l,n of the first block, the charge generated in each light-receiving element P.sub.l,j of the first block is transferred to and stored in the corresponding wiring capacitance C.sub.L. The charges stored in the wiring capacitances C.sub.L change the potentials of the common signal lines 104, and these potentials are sequentially output to corresponding output lines 107 by sequentially turning on analog switches SW.sub.j arranged in the driver IC 105. Then, by turning on the switching elements T.sub.2,l -T.sub.2,n to T.sub.k,l -T.sub.k,n of the second to kth blocks by gate pulses .phi.G.sub.2 to .phi.G.sub.k, respectively, the charges generated by the light-receiving elements P.sub.2,l -P.sub.2,n to P.sub.k,l -P.sub.k,n are transferred on a block basis.
That is, when the switching elements T.sub.l,l -T.sub.k,n are on/off-controlled through the control lines G.sub.l -G.sub.k, the n switching elements of one block are simultaneously controlled, and the signals from the n light-receiving elements are introduced to the driver IC 105 in parallel. The potentials of the common signal lines 104 due to the transferred charges are sequentially read out, so that an image signal for a single line in the main scanning direction of a document can be obtained. The document is moved by a document feeding means (not shown) such as a roller while repeating the above operation, to thereby obtain an image signal of the entire document (refer to Japanese Patent Unexamined Publication No. Sho. 63-9358). Switches RS serve to remove a residual charge in the respective wiring capacitances C.sub.L (resetting).
In the image sensor as described above, the on/off control of the switching elements is performed by switching a voltage V.sub.G, which is applied to the control line G.sub.i from the TFT control circuit 106, between two levels: high and low. Referring to FIG. 10, which shows an equivalent circuit for a single bit part of the image sensor, a change of the voltage V.sub.G affects the potential of the capacitance C.sub.L through the overlap capacitance C.sub.GS of the switching element T. That is, if the on/off potential difference of the voltage V.sub.G is expressed as .DELTA.V.sub.G, the potential change in the wiring capacitance C.sub.L becomes C.sub.GS .DELTA.V.sub.G /(C.sub.L +C.sub.GS). If the wiring capacitance C.sub.L is sufficiently large, such a potential change is negligible. However, if the wiring capacitor C.sub.L is small, it has some appreciable effect. This potential change causes an increase of an offset potential of the common signal line 104, which results in the difficulties in dealing with the signals in the driver IC 105 and the subsequent stages.